Combine egg yolks with half of the sugar and whisk to a light ribbon consistency. Next mix in the flour, no lumps please.
In a saucepan scald milk with vanilla and the rest of the sugar. Pour hot milk over the egg yolks, stirring all the time. Mix well and return to the saucepan.
Bring the mixture to boil over medium-low heat, stirring continuously. Allow to bubble for a couple of minutes then tip into a bowl and stir in the butter. Cover with cling film to prevent a skin from forming.

Sieve flour into a
bowl. Heat milk to 38°C. Mix yeast with sugar and warm milk. Pour into the
flour, add cardamom, salt, egg white and butter. Mix it all together and knead
into a smooth dough. About 7 minutes with a machine or a bit longer by hand.
Cover and let rise in a warm place for about 50 minutes.

Divide the dough in half.

For the poppy seed buns:
Roll half of the dough into a shape of a rectangle. Spread with one-third of the pastry cream and scatter with plenty of poppy seeds. Now, with the longer side facing you, roll it up into a log. Using a clean wet knife, cut the log into slices, about 2cm thick. Should yield about five or six. Place the slices on a baking tray, pressing them into shape with your fingers. They might look lopsided after cutting. Drop a spoonful of pastry cream in the centre of each bun, cover and let rise. After rising, brush edges with sweetened milk and sprinkle with additional poppy seeds.

For the rhubarb buns:
Divide the other half of dough
into five equal pieces. Briefly kneading each piece between your palms, roll the
bits of dough into beautiful spheres and press into a flat circle, leaving the edges just slightly thicker. Scoop a good portion of pastry cream on top of each dough circle, about two-three tablespoons. Top with poached rhubarb. Place on a baking tray, leaving enough
space between them. The buns will be double the size after rising and baking.
Cover the tray and leave to rise in a warm place for 30-40
minutes. After rising, brush the edges of the buns with sweetened
milk.

Preheat the oven to 210°C. Bake buns in the oven for 8 minutes. Do keep an eye on them as they will
brown quickly. If the buns are not done after about eight minutes, then the oven
is not hot enough. If the buns seem to get burnt then the oven is too hot.